1. Field of the Invention
The present invention relates to coated fibers and cables prepared therefrom and, more particularly, to hydrolytically stable, thermoplastic, polyurethane-coated, multifilament strength members for said cables.
2. Description of the Prior Art
Quite often in power and communication cables, the electrical conductor also serves as the strength member, providing the necessary mechanical support as well as the electrical transmission path. For many applications, however, the electrical conductor cannot provide the necessary mechanical strength and protection that are required, and must be joined together with separate strength members. Such cables, which obviously have a significant percentage of their volume composed of strength members, are normally referred to as electromechanical cables which are externally armored to provide both strength to support the weight of the cable and mechanical protection against abrasion and cutting.
Typical oceanographic missions for electromechanical cables include the launch, recovery and control of tethered vehicles, the power and control for mining or bottom sampling equipment, towed instrumentation sleds or bottom-mounted static arrays. The electrical portion of these cables is used to transmit communication signals, control signals, and sensor data, and for power transmission to equipment installed on the ocean floor or suspended in the water column.
The analysis and design of the mechanical portion of the cable, and its influence on the electrical properties, is a well developed science. For cables deployed from a ship, an accurate prediction of motions and loads is difficult, if not impossible. Since mechanical failure will generally mean the loss of expensive equipment and potential injury to personnel, cable designers are forced to be extremely conservative. This, coupled with the fact that until recently steel was the only choice available as a reliable strength member material, meant that long cables would have high self-weight. From a systems viewpoint, this relfected a decrease in convenience and ease of operations, and a definite increase in the size and cost of associated handling gear.
Bending fatigue, from repeated flexing of cables under load over a sheave, is another mechanical problem of great concern to the designer. High-strength steel has relatively poor flexure fatigue resistance, but other materials have not been available as an alternative. As longer cables are required for deeper application, the high self-weight of the strength members produces an uncomfortably low static factor of safety, aggravating the already serious fatigue problem. The use of lightweight synthetic strength members has generally not been acceptable, due to their low elastic modulus which is not compatible with the low allowable stretch of electrical conductors incorporated in the cable.
Steel and titanium were generally unacceptable because of their low strength-to-weight ratios and poor fatigue properties under flexure. Boron and graphite appeared attractive initially, because of their high strength-to-weight ratios and high modulus, but poor abrasion resistance and extremely high cost eliminated them as practical solutions. Fiberglass had been used successfully in other lightweight marine cable applications but suffered from abrasion problems as well as a susceptibility to static tensile fatigue.
Recently a new, synthetic, organic, high modulus material has become available having a higher modulus than fiberglass, lower density, better abrasion resistance, equal or better strength and better static tensile fatique properties. A protective coating is necessary:
(1) to isolate the fibers and protect them from destructive self abrasion;
(2) provide load adjustment from fiber to fiber or to provide load normalizing when the fiber bundle or yarn is loaded in tension;
(3) to protect the fibers from hostile environments of harmful chemicals such as strong acids, ultraviolet radiation or abrasive particles such as sand; and
(4) to make it possible to form or preform the coated yarn or fiber bundle so that it will retain all or part of the shape change imposed on the coated yarn. This characteristic is important to making rope and other load carrying line products.
Attempts to impregnate the fibers with epoxy or urethane resins were unsuccessful. Epoxy resins must have a 25% matrix for maximum load capability and 35-40% for peak load strength. Even utilizing silicone as a lubricant for inter-fiber slippage as the cable is flexed, the rigid epoxy coating prevented fiber movement. The hydrolytic stability of epoxies in sea water is questionable. When it was attempted to impregnate the fibers with a polyurethane (Estane 53800), the results were again unfavorable due to poor fiber wetting and incomplete penetration of the fiber bundles